html,
body,
.main {
    height: 100%;
}

.main {
    display: flex;
    flex-direction: column;

    .topbar {
        width: 100%;
        height: 50px;
        // background-color: yellow;
        position: fixed;
        top: 0;
        left: 0;
        z-index: 100;
    }

    .box {
        flex: 1;
        // background-color: blue;
        padding-top: 50px;
    }
}